1. A basic expression consists of either
- A relation in the database.
- A constant relation.
2. General expressions are formed out of smaller subexpressions using
- Σp(E1) select (p a predicate)
- â…¡s(E1) project (s a list of attributes)
- ρx(E1) rename (x a relation name)
- E1 ∪ E2 union
- E1 -E2 Set difference
- E1 × E2 cartesian product